And now you are terribly wrong. This document explicitly prohibits use of M900 in tanks other than Abrams; "M1 only" superseded by capital bold "Warning!" have very straight meaning "Never ever try!!!"(just because word "Warning!", if used in manuals always means direct hazard to operator's life or health- this is international standard for english-languaged technical documentation).
As for foreign users. First, M900 is hardly an exportable round because of DU penetrator. Second- foreign users of M60A3 are generally using translations of US manuals, or local compilations based on original american documentation. Third- if foreign user desires to use non-standard ammo, this is possible only if evaluation(in fact a small research study involving engineering analysis and damn lot of tests, which can take a lot of time) is successful, e.g. new ammo is safe to use, no compatibility issues, specifications claimed by manufacturer are met and so on. Ammo is not being fielded just because it looks sexy in promo flyer.